With the proliferation of small arms and global terror, the need for ballistic countermeasures has become increasingly acute. Several alternate approaches to solving the projectile detecting and/or tracking problem have been developed. Most employ acoustic or electro-optic (EO) sensors to solve for the trajectory vector.
Squire describes a system that uses an infrared camera to first detect a bullet via its thermal signature and then switches to a tracking mode using a laser radar. Back projection algorithms are then used to derive the projectile velocity vector.
Rowland describes a system that applies acoustic or optical sensors to determine the trajectory and source of the projectile. These types of systems exploit the amplitudes and arrival times of the so called N-wave (the acoustic signature of a supersonic projectile) at known sensor positions that varies as functions of the distance or range to the trajectory of the projectile.
McNelis presents an alternate acoustic apparatus for determining the trajectory of a supersonic projectile of unknown velocity and direction. In this apparatus, at least three displaced sensors are capable of encountering a shock wave generated by a supersonic projectile passing in the vicinity of the sensors. The sensors generate signals in response to the shock wave that are related to an azimuth and elevation angle of a unit sighting vector from each sensor to an origin of the shock wave.
Systems have also been proposed that rely on radar. The patents to Stevens, et al., which include U.S. Pat. Nos. 6,029,558 and 6,412,391, reside in a counter-terrorism, reactive personnel protection system which detects the presence of a concussive shock wave or ballistic projectile as it approaches a designated personnel target. Before impact, an air bag is rapidly inflated and interposed between the destructive force and the target so as to provide a protective barrier. The air bag is constructed from ultra-high molecular weight polyethylene material, and serves to halt or redirect the detected destructive force and thereby protect the designated target from attack.
In the case of projectile detection and protection, the Stevens' patents propose a radar-based bullet detection system with anti-jamming electronics is used to detect the presence of an incoming small arms projectile and determine its path of travel. A bi-static radar system is used to detect the Doppler shift signature to reliably determine the presence of a bullet, and discriminate between the bullet and any other rapidly moving object in the vicinity. Additionally, signal processing circuitry and algorithms are used to help differentiate between projectiles and noise or other extraneous signals to prevent false alarms. Once the presence of a ballistic object is confirmed, a control unit activates a gas generation device, which in turn rapidly inflates an anti-ballistic air bag.